Manufacture of nuclear fuel elements

ABSTRACT

A method of agglomerating particles of nuclear fuel resides in tumbling the particles in a drum or other container while spraying alternately in succession powdered graphite/synthetic resin mixture and a spray of solvent for the resin wherein the proportions of graphite powder may be between 60 and 40 percent of the total powder mix.

United States Patent Gough et al.

[ 1 June6,1972

[54] MANUFACTURE OF NUCLEAR FUEL ELEMENTS [72] Inventors: John Richard Cox Gough, Weymouth, Dorset; Geoffrey William Meaden, Wareham, Dorset; Michael Stuart Thomas Price, Weymouth, Dorset, all of England [73] Assignee: United Kingdom Atomic Energy Authority,

London, England [22] Filed: Dec. 2, 1968 211 Appl. No.: 780,602

[30] Foreign Application Priority Data Apr. 22, 1968 Great Britain ..18,985/68 U.S. Cl ..264/0.5, 176/89 Field of Search ..264/O.5

Primary Examiner-Carl D. Quarforth Assistant Examiner-S. Hellman Attorney-Larson, Taylor and Hinds [57] ABSTRACT A method of agglomerating particles of nuclear fuel resides in tumbling the particles in a drum or other container while spraying alternately in succession powdered graphite/synthetic resin mixture and a spray of solvent for the resin wherein the proportions of graphite powder may be between 60 and 40 percent of the total powder mix.

2 Claims, No Drawings MANUFACTURE OF NUCLEAR FUEL ELEMENTS BACKGROUND OF THE INVENTION This invention relates to the manufacture of nuclear fuel elements incorporating fuel particles bearing an outer layer of fission product retaining material. Various methods of incorporating these coated particles into fuel elements have been proposed hitherto. According to some of these methods the fixing of a mass of particles into an agglomerate is achieved by overcoating the coated particles with some material which acts as a binding agent to hold the particles together. In one such process described in copending U.S. Pat. application Ser. No. 630,408, now US. Pat. No. 3492379 the coated particles are first overcoated with a layer of a therrnosetting synthetic resin and then poured into a suitable cavity wherein the particles are heated to fix them into position. By this means a high packing density of particles could be achieved within a given volume while individual particles could not easily be detached from the mass inadvertently and go astray. According to the example described in the aforementioned copending patent application, this overcoating was applied by tumbling some coated particles in a drum into which a powdered mixture of phenolformaldehyde resin softened in a spray of solvent was supplied. A small quantity of resin-treated, powdered, graphite, (up to 20 percent by weight of the resin) was mixed with the resin in order to render the resin free flowing as a powder. Thus the overcoating was mainly of resin with a modicum of graphite.

Now it has been found that, for some conditions of use, fuel agglomerates manufactured according to these prior proposals can fall short of what is desired. For example, the bond between adjacent particles which are virtually in contact with one another, or else separated by a very thin film of resin, can easily fracture either during manufacture due to the extensive contraction of the synthetic resin during carbonization or when the expected stresses and strains clue to dimensional change occur in operation.

SUMMARY OF THE INVENTION According to the present invention we have found that if the proportion of powdered graphite is increased to above 20 percent and up to not more than 70 percent by weight of the resin powder then a considerably stronger agglomerate is achieved. To this end, the invention provides a process in which particles of fission product retaining fuel are coated over with a layer of a substance derived from a powdered synthetic resin and graphite in admixture, such that graphite constitutes between 20 and 70 percent by weight of the resin present in the mixture. A preferred range lies between 40 and 60 percent by weight of graphite powder, the remainder being synthetic resin. Particles overcoated by this modified process are more suited to form coherent artifacts or agglomerates by a not inconsiderable degree than those overcoated by prior art methods. The apparently small change in content of graphite powder in the overcoating ensures a buffer region between particles which enables the resultant agglomerate to withstand strain well and, also, to retain its particles in place.

However the performance of this method is not an easy one and if it is to be performed economically and as a practical proposition without causing the distribution of powder and solvent droplets throughout the whole area in which the process is being carried out then the following procedure should be followed. That is to say, the powder mixture and the solvent spray should be introduced in succession alternately and at no time should both powder and solvent spray be introduced simultaneously.

The process preferentially involves the tumbling of particles in a rotating drum with the resin-graphite powder mixture dusted on to the particles from a vibrating sieve. Every few seconds the vibration of the sieve is halted so that the feed of powder is stopped and a short burst of a solvent for the resin, is sprayed into the drum. After the burst of solvent the feed of powder is continued with the intermittent spraying until the particles have attained a layer of the correct thickness. The alternate supply of powder and solvent is then stopped and tumbling continued until the particles are hard and dry. The particles are now ready for forming into an agglomerate either by a simple moulding process or by being poured into a cavity with or without vibration in a graphite body and cured in situ.

When made into shaped tubular agglomerates, however, by such moulding methods, the moulded body tends to crack longitudinally. Now, in this connection we have determined that cracking is most likely due to the resistance offered to shrinkage of the annular body by the inner annular wall of the mould. According to a further feature of the invention, a method of forming an annular fuel body which comprises essentially an annular agglomeration of coated particles separated from each other and bonded to one another by a layer ofgraphite/resin mixture resides in spacing the particles from the inner wall of the mould which is to define the bore of the annular fuel body by a layer of a filmy material which remains in place during polymerization of the resin and will thereafler decompose without substantial residue.

To exemplify the invention a method of forming a tubular agglomeration of coated particles within a graphite tubular fuel container will now be described.

A batch of 750 grams of nuclear fuel particles each about 1,100 microns in diameter and previously coated with fission product retaining material was placed in a rotary drum mounted for rotation on an axis which is inclined at a small angle to the horizontal and having its upwardly facing end open for the introduction of the process materials. The drum was rotated at 30 r.p.m. on its axis. A supply of powder composed of a mixture, in equal proportions by weight, of powdered graphite and phenolformaldehyde resin powder was supported in a vibratory feeder above the tumbling bed of fuel particles and, quite independently of this, a spray injector for methylated spirit was made available.

The feeder was then vibrated in order to feed the mixed powders into the drum. Intermittently, at intervals of a few seconds, the vibratory feeder was stopped so that the introduction of powder ceased and the spray injector was then operated to inject a spray of methylated spirit into the drum. During the continuous rotation of the drum, the introduction of mixed powders into the drum was repeatedly interrupted for the injection of the solvent spray, so causing an overcoating of the mixture to build up on the tumbling particles. When suflicient powder mixture had been delivered to build up the correct thickness of overcoat on the particles, that is, when the powder mixture supplied amounted to 6 percent by weight of the particles, the supply of process materials was stopped. The rotation of the drum was continued to dry off any residual solvent and was then stopped.

The particles were removed from the drum and, in an uncured state, vibrated into the annular cavity of a doublewalled tubular container. The container was of graphite and formed by two coaxial tubes of appropriately differing diameters to define an annular cavity of a thickness corresponding to the intended wall thickness of agglomerate. To prevent longitudinal cracking on subsequent carbonization of the agglomerate, a film of 0.005 inch thick polythene was wrapped around the inner face of the cavity.

The container and its contents were heated to 200C in air to polymerize the resin content of the overcoating powder and fix the particles in position. The container was then placed in a furnace purged by a stream of inert gas and heated to 800C during which process the resin is carbonized. A final heat treatment was carried out at 1,800C in vacuum to degas the agglomerate. The container was then closed by a plug and was ready for use as one fuel bearing component of a fuel element. Radiographic inspection showed that the polythene film had decomposed during the heat treatment with insignificant carbon residue so leaving an annular clearance between the agglomerate and the inner annular face of the container. This allowed the agglomerate to contract towards the axis of the container without cracking longitudinally.

As an alternative to forming an agglomerate of fuel particles in situ within a fuel container for use directly in a nuclear reactor, an agglomerate may be similarly formed in a mould from which the product is removed after the carbonizing or the degassing heat treatment step. The clearance formed in the bore of the agglomerate by the decomposition of a polythene film, as before, facilitates removal of the agglomerate from the mould.

We claim:

1. A method of forming a body of nuclear fuel incorporating fission product retaining nuclear fuel particles which have bee provided with an outer coating of fission product retaining material comprising the steps of applying to the outer surfaces of such particles a further coating comprising powdered graphite and resin in proportion 40 to 60 percent graphite to 60 to 40 percent resin by tumbling the particles in a drum while intermittently and sequentially and not simultaneously introducing the powder and solvent into the drum and, once the coating is applied, heating a mass of particles in contact with one another to form a nuclear fuel body comprising agglomerated particles.

2. A method of forming a nuclear fuel element as claimed in claim 1 in which the fuel element is of annular form which resides in preparing an annular cavity having coaxial cylindrical surfaces, applying a film of synthetic plastic material around the inner of said surfaces filling the cavity with particles bearing said further coating and then heating the particles in the cavity. 

2. A method of forming a nuclear fuel element as claimed in claim 1 in which the fuel element is of annular form which resides in preparing an annular cavity having coaxial cylindrical surfaces, applying a film of synthetic plastic material around the inner of said surfaces filling the cavity with particles bearing said further coating and then heating the particles in the cavity. 